Front page | perl.perl5.porters |
Postings from December 2013
On Mon, Dec 02, 2013 at 04:34:34PM +0000, Dave Mitchell wrote:
> On Mon, Dec 02, 2013 at 03:42:59PM +0000, Zefram wrote:
> > Dave Mitchell wrote:
> > > I fixed this by stopping the processing of the integer or fractional
> > > component once its been detected that only zeroes are left (with a
> > > suitable adjustment of the exponent).
> >
> > Eww. That won't fix cases where the extra digits are non-zero, such as
> >
> > $ echo 0.15301e-305 | perl -e '$v = <STDIN>; print "v=", $v + 0, "\n";'
> > v=0
> > $ echo 0.15399e-305 | perl -e '$v = <STDIN>; print "v=", $v + 0, "\n";'
> > v=0
>
>
> Oh yeah :-(
>
> I'll need to rethink that.
Second attempt now smoking as smoke-me/davem/float1:
commit 72cfc3028683d7d6428ee0b881a788aba667fe8e
Author: David Mitchell <davem@iabyn.com>
AuthorDate: Mon Dec 2 15:04:49 2013 +0000
Commit: David Mitchell <davem@iabyn.com>
CommitDate: Tue Dec 3 12:30:01 2013 +0000
[perl #120426] atof() small value rounding errors
For something like 0.153e-305, which is small, but not quite the smallest
number (which is around 2.2e-308), adding extra digits to the fractional par
could cause unnecessary rounding to zero.
From the bug report:
$ echo 0.1530e-305 | perl -e '$v = <STDIN>; print "v=", $v + 0, "\n";'
v=0
$ echo 0.153e-305 | perl -e '$v = <STDIN>; print "v=", $v + 0, "\n";'
v=1.53e-306
This was because 0.1234e-305 is calculated as
1234 / (10^309)
and 10^309 becomes infinity. In these edge cases, repeatedly decrement
the exponent and divide the mantissa by 10 until the exponent becomes in
range; in this case we instead calculate
123 / (10^308)
--
Technology is dominated by two types of people: those who understand what
they do not manage, and those who manage what they do not understand.
Thread Previous
|
Thread Next